The brown marmorated stink bug (Halyomorpha halys) is an invasive species widespread across North America. Its annual cycle is strictly dictated by the seasons, involving distinct phases of outdoor feeding and reproduction followed by a period of indoor dormancy. Understanding when and why this shield-shaped insect appears and disappears relates directly to temperature and its biological need to survive the colder months.
Spring and Summer: The Outdoor Active Season
The disappearance of cold weather triggers adult stink bugs to emerge from overwintering sites, typically beginning in late March or April. Sustained warmth above 50–60°F prompts them to leave shelter and seek food outdoors. Once active, adults mate, and females lay clusters of eggs, often on the underside of leaves.
The eggs hatch into nymphs that progress through five developmental stages, or instars. Development takes approximately 40 to 60 days, depending on temperature and daylight hours. Throughout this period, the insects actively feed on a wide variety of plants, including fruits, vegetables, and ornamental trees, building up energy reserves for the coming winter.
Autumn: Seeking Shelter Indoors
Stink bugs become most noticeable to homeowners starting in late summer and accelerating into autumn. This behavioral shift is driven by two main environmental cues: the shortening of daylight hours (photoperiod) and the first consistent drop in nighttime temperatures. As temperatures cool, adult insects begin overwintering migration, aggregating and searching for protected sites.
This migration commonly begins in late August and peaks through September and October. The insects are migrating not for food, but to find a dry, secluded location where they can enter a state of suspended development. They are attracted to vertical structures, like trees and buildings, which offer protection from the elements.
Homes and other human-made structures provide ideal overwintering spots because they offer stable temperatures that prevent freezing and reduce the risk of desiccation. The insects enter through small openings, such as cracks in the foundation, gaps around utility conduits, window and door frames, and attic vents. Once inside, they typically move deep into wall voids, attics, or crawl spaces, seeking a tight, dark space to settle for the winter.
Winter: Diapause and Dormancy
Once inside sheltered locations, adult stink bugs enter diapause, a biological state of dormancy. During diapause, the insect’s metabolism slows significantly, and reproductive activity ceases as it conserves stored energy. Only the adult stage is capable of entering this overwintering state, ensuring they are ready to emerge and reproduce immediately when spring returns.
The bugs remain largely inactive throughout the coldest months, hidden deep within the structure of a building. A sudden, prolonged warm spell can sometimes briefly rouse them from diapause, causing a few individuals to appear inside a living space near a heat source or window. This accidental emergence is temporary; they will return to a sheltered state or perish unless the warmth signals the true arrival of spring.